Google wouldn’t disclose exactly why they phased it out, though it seems obvious that they’re paving the way for Google+ profile searches. When Google+ launched, instead of adopting Twitter’s @reply syntax, they coined their own format for mentioning people — adding a plus to the beginning of a name — triggering the future conflict with the + operator.

Google hasn't said, but the speculation amongst the Technorati is that it's related to Google+. Now that they've got The Plus and they're integrating it everywhere, they're likely reserving it for some specialized search related to Google+.

We've made the ways you can tell Google exactly what you want more consistent by expanding the functionality of the quotation marks operator. In addition to using this operator to search for an exact phrase, you can now add quotation marks around a single word to tell Google to match that word precisely. So, if in the past you would have searched for [magazine +latina], you should now search for [magazine "latina"].

We're constantly making changes to Google Search - adding new features, tweaking the look and feel, running experiments, - all to get you the information you need as quickly and as easily as possible. This recent change is another step toward simplifying the search experience to get you to the info you want.